I realize this is an older thread, but I this is the only conversation I could find regarding NetGuard and this setting.
Spoke to Developer, and their recommendation to keep the app working well is to use alternative URL: www.opendns.com
Hope this helps some people.
Thank you so much.
The question was:
It took 322 days to get the answer, and a lot of bitching in between.
At least it was worth it to read until the end, though it had an absurd touch, as if Pinter was here.
Have a good time.
In TrackerControl www.f-droid.com is configured instead of google.com
That’s OK for me. And maybe for you, too. 
It seems all about insecurity from the beginning.
If you don’t know whether to change anything without messing things up - in this case it could affect the https-connection - you falter.
I installed NetGuard yesterday and stumbled over the preset GRGL-site. Since I tend to get rid of any possible data-provision there I started searching.
No big deal makes a small deal.
I do want to avoid any deal with them.
I set it to www.opendns.com.
So, if any website expectedly online could do the job: allright.
But is it?